BUTOVO, Russia (Reuters) - Russian President Vladimir Putin paid his respects on Tuesday to millions of people killed under Soviet dictator Joseph Stalin and called for the country to unite to prevent a repeat of its tragic past.

Putin, a former KGB spy, marked Russia's annual day of remembrance for the victims of Stalin's purges with a visit to Butovo, a military training ground near Moscow where tens of thousands of people were executed by firing squads.

Millions of people were executed under Stalin and many more perished from abuse and disease in a vast network of prison camps, known as the Gulags.

The victims included priests and royalists but also huge numbers of people who were simply caught up in an indiscriminate spiral of killing. This year Russia marks the 70th anniversary of the bloodiest period of the purges.

Putin attended a memorial service with Patriarch Alexiy II, head of the Russian Orthodox Church, after passing a field criss-crossed with mass graves.

"We know very well that 1937 was the peak of the purges but this year was well prepared by years of cruelty," Putin said beside a mass grave after laying flowers at a memorial.

Putin said such tragedies "happen when ostensibly attractive but empty ideas are put above fundamental values, values of human life, of rights and freedom."

"Hundreds of thousands, millions of people were killed and sent to camps, shot and tortured," he said. "These were people with their own ideas which they were unafraid of speaking out about. They were the cream of the nation."

In an appeal for national unity, Putin said: "To develop the country and choose the right path, we need political debates and even battles but to make this process creative they should not be conducted outside the cultural framework," Putin said.

Historians estimate that between 20 million and 40 million died during Stalin's rule, tearing families apart and creating a climate of fear that haunted the Soviet Union.

Also on Tuesday, dozens of mainly older Russians laid flowers at a stone memorial outside the headquarters of the former KGB -- now known as the Federal Security Service -- to remember Stalin's victims.

GREAT TERROR

Stalin, who succeeded Vladimir Lenin, started a series of purges in the 1930s that became known as the Great Terror. The NKVD security service, the predecessor to the KGB, killed hundreds of thousands of people on trumped up charges.

Butovo was just one of hundreds of killing grounds. More than 20,000 people are known to have been executed there between August 1937 and October 1938 alone, though local priests say the figure could be as high as 60,000.

"According to documents we have seen, most of the people shot here were peasants and workers, but there were many dignitaries as well," said Deacon Dmitry, a priest at the site.

"There was even a complete theatrical troupe from the Baltics massacred here," he said.

(Additional reporting by Chris Baldwin)